Low Voltage Controller for Touch Screens
AD7879
FEATURES
4-wire touch screen interface
1.6 V to 3.6 V operation
Median and averaging filter to reduce noise
Automatic conversion sequencer and timer
User-programmable conversion parameters
Auxiliary analog input/battery monitor (0.5 V to 5 V)
1 optional GPIO
Interrupt outputs (INT, PENIRQ)
Touch-pressure measurement
Wake-up on touch function
Shutdown mode: 6 μA maximum
12-ball, 1.6 mm × 2 mm WLCSP
16-lead, 4 mm × 4 mm LFCSP
FUNCTIONAL BLOCK DIAGRAM
V
CC
/REF
X– Y– X+ Y+
X+
X–
REF–
Y+
6-TO-1 MUX
REF–
REF+
Y–
GND
TEMPERATURE
SENSOR
12-BIT
SAR ADC
FILTERING
RESULT
REGISTERS
PENIRQ/INT/DAV
07667-001
AUX/VBAT/GPIO
AD7879/
AD7879-1
CONTROL
REGISTERS
APPLICATIONS
Personal digital assistants
Smart hand-held devices
Touch screen monitors
Point-of-sale terminals
Medical devices
Cell phones
SERIAL PORT
TO
RESULT
REGISTERS
CS/
DIN/ DOUT/ SCL
ADD0 ADD1 SDA
SEQUENCER
AND TIMER
Figure 1.
GENERAL DESCRIPTION
The AD7879 is a 12-bit successive approximation analog-to-
digital converter (ADC) with a synchronous serial interface and
low on-resistance switches for driving 4-wire resistive touch
screens. The AD7879 works with a very low power supply (a
single 1.6 V to 3.6 V) and features throughput rates of 105 kSPS.
The device includes a shutdown mode that reduces its current
consumption to less than 6 μA.
To reduce the effects of noise from LCDs and other sources,
the AD7879 contains a preprocessing block. The preprocessing
function consists of a median and an averaging filter. The com-
bination of these two techniques provides a more robust solution,
discarding the spurious noise in the signal and keeping only
the data of interest. The size of both filters is programmable.
Other user-programmable conversion controls include variable
acquisition time and first conversion delay; up to 16 averages
can be taken per conversion. The AD7879 can run in either
slave or standalone mode, using an automatic conversion
sequencer and timer.
The AD7879 has a programmable pin that can operate as an
auxiliary input to the ADC, as a battery monitor, or as a GPIO.
There is also a programmable interrupt output that can operate
in three modes: as a general-purpose interrupt to signal when
new data is available INT, as an interrupt to indicate when limits
are exceeded, or as a pen-down interrupt when the screen is
touched (PENIRQ). The AD7879 offers temperature measure-
ment and touch-pressure measurement.
The AD7879 is available in a 12-ball, 1.6 mm × 2 mm WLCSP
and in a 16-lead, 4 mm × 4 mm LFCSP. The part also has either
an SPI (AD7879) or I
2
C (AD7879-1) interface.
Rev. 0
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